1. Field of the Invention
This invention relates to a method for preparing a salt of 4-hydroxystyrene by reacting 4-acetoxystyrene with a suitable base in a suitable solvent system; in addition, the method pertains to preparation of 4-tertiary-butoxycarbonyloxystyrene from the salt of 4-hydroxystyrene.
2. Description of the Prior Art
4-Hydroxystyrene is a well-known compound which is itself useful as a food flavoring substance and as an intermediate in the preparation of polymers and copolymers useful in coatings, electronic applications, ion exchange resins, photoresists, etc.
Although there are several ways to prepare 4-hydroxystyrene these known methods are not commercially feasible in the further utilization of 4-hydroxystyrene, such as for example for use in the preparation of a photoresist intermediate such as 4-tertiary-butoxycarbonyloxystyrene. The reason for this is that the 4-hydroxystyrene itself is difficult to isolate, in that it (1) readily decomposes, (2) is toxic via skin absorption, and (3) readily polymerizes. A preparation for 4-hydroxystyrene utilizing 4-acetoxystyrene is reported in a paper entitled "Preparation of Vinyl-phenols and Isopropylphenols", Corson et al., Volume 23, Apr. 1958 J. Org. Chem. In this preparation, 4-acetoxystyrene is saponified in an aqueous system with a large concentration of a soluble base, KOH, to produce an aqueous solution of the potassium salt of 4-hydroxystyrene which is neutralized with acid to precipitate 4-hydroxystyrene. As indicated above, the procedure is not practical or commercially feasible for production of large quantities of 4-hydroxystyrene because the 4-acetoxystyrene and/or the 4-hydroxystyrene is not very stable and readily polymerizes under the aqueous saponification conditions employed therein which involve high concentrations of soluble base, resulting in poor yields of 4-hydroxystyrene. A more efficient process for producing 4-hydroxystyrene from 4-acetoxystyrene is desired and needed. The instant invention provides a method whereby such ready polymerization of the 4-acetoxystyrene and/or the 4-hydroxystyrene in the formation of 4-hydroxystyrene is avoided.
4-Tertiary-butoxycarbonyloxystyrene is useful as an intermediate in the production of the photoresist material poly(4-tertiary-butoxycarbonyloxystyrene). The only published route to 4-tertiary-butoxycarbonyloxystyrene is reported by Frechet et al., Polymer, 1983, 995 and Ito et al., U.S. Pat. No. 4,491,628, which involves a Wittig reaction starting with p-hydroxybenzaldehyde. The formation of 4-tertiary-butoxycarbonyloxystyrene directly from p-hydroxystyrene has never been publicly reported, most likely due to the fact of the great instability of the 4-hydroxystyrene, as discussed above, and the difficulties most likely encountered thereby. These difficulties would lead one skilled in the art away from trying such synthesis using 4-hydroxystyrene even in the face of the synthesis of 4-tertiary-butoxycarbonyloxy-.alpha.-methylstyrene from 4-hydroxy-.alpha.-methylstyrene, as reported in U.S. Pat. No. 4,491,628. The reason for this is the fact that any .alpha.-substitution at the site of a carbon-carbon unsaturated bond lends great stability to that site and thereby resistance to polymerization, as compared to the unsubstituted site of the styrene unsaturation of 4-hydroxystyrene.